The aesthetic properties of alkyl polyglycoside solutions are apt to erode over a short period of time due to the crystallization of the alkyl polyglycoside. Once the alkyl polyglycoside solutions crystallize, the viscosity of the solutions increases to the point where handling problems result due to the non-homogeneity of the product. The crystallization phenomenon in fats and lipids is well known. All fats and lipids commonly form lamella-type structures, but every substance reveals different unit cell structures, i.e., polymorphism. The crystal structure of each modification depends on the geometrical conformation and chemical bonding of a molecule in which molecular interactions differ between different segments, such as an aliphatic chain, an olefin group in unsaturated fatty species, a methyl end group which stabilizes the lamella-lamella interface, a glycerol group in acylglycerol, --COOH in fatty acid, a polar head group in polar lipids, etc. Strongly hydrated nonionic surfactants have phase diagrams resembling those of ionic surfactants, suggesting the presence of a strong repulsive force between the micelles. These strong hydration repulsion forces are oftentimes balanced by van der Waals attractive forces between the lipid bilayers which provide for a uniform mixture.
It is well known that at a temperature below the critical transition temperature, a surfactant-water mixture exists in the so-called coagel and gel states, where the hydrocarbon chains of surfactant molecules are in a trans zigzag elongated state. A difference between the coagel and gel phases can be clearly recognized by the naked eye. That is, the gel phase is in a homogeneous, semi-transparent state, while the coagel phases consist of a hydrated-crystalline state separated from the water solution phase. This is considered to be due to the difference in the mode of binding forces operating in the polar head region in the two phases. Probably, the predominant binding force in the coagel phase is electrostatic interaction between cationic head groups and their counter-ions, while there are in the gel phase hydration interactions of both with water molecules.
Physical deterioration of fat products such as margarine, shortening, and chocolate, just to name a few, depends on the size, morphology and polymorphic structure of the fat crystals, all of which are primarily influenced by the crystallization conditions, and secondly, by the phase transformation.
Kinetic parameters in the crystallization processes involve molecular clustering, solvation/desolvation, adsorption, surface/volume diffusion, conformational rearrangements and so on.
Crystallization involves two elementary processes: nucleation and crystal growth. Nucleation occurs when the solution or melt phases deviate from thermodynamic equilibria to a greater extent. Three actual nucleation phenomena occur in real systems. They are homogeneous nucleation which occurs in a very pure system and at a higher driving force. Heterogeneous nucleation predominates either in an impure system or at a lower driving force. Secondary nucleation which becomes important when the pre-existing crystals provide secondary nuclei so that the precipitation is remarkably enhanced.
Certain alkyl polyglycosides have a tendency to crystallize during storage at ambient temperatures due to their supersaturated state. Upon crystallization, the alkyl polyglycosides become turbid, i.e., cloudy and opaque in appearance due to the solids contained therein falling out of their supersaturated state which, when coupled with the resultant non-homogeneous state of the crystallized solution and the attendant difficulties in handling operations such as pumping, is a highly undesirable phenomenon. In order to rectify this undesirable physical state, the alkyl polyglycoside composition must be heated in order to resolvate the crystals. As can be imagined, this process is both expensive and time-consuming.
Another problem associated with alkyl polyglycosides relates to the undesirable tactile properties which they impart upon contact with the human body, commonly referred to in the industry as "scroopiness". More particularly, alkyl polyglycosides, when applied onto hair and/or skin tend to make it feel rough, dry and sticky as if hairspray were applied thereon. In general, alkyl polyglycosides, due to their nonionic character, synergistic relationship with other surfactants, tendency towards high foaming and mildness with respect to skin irritation, have become highly desirable surfactants for use in the personal care products industry. However, due to their tendency to impart this highly undesirable scroopiness effect when in contact with the human body, their use in the personal care industry, in significant amounts, has been limited.
Typical commonly used hydrotropes such as ethanol and SXS have proven to be ineffective for maintaining the homogeneity of alkyl polyglycoside solutions, and thus inhibiting crystallization.
Consequently, it is the primary object of this invention to provide a composition and process of enhancing the aesthetic properties of alkyl polyglycosides by either reducing or eliminating their tendency to crystallize during prolonged storage under ambient conditions.
It is also an object of this invention to eliminate scroopiness in human hair and/or skin as a result of treatment with cleansing compositions containing alkyl polyglycosides.